Frequency stabilizer for oscillators



Oct. 18, 1949. w, E, BRADLEY 2,485,029

FREQUENCY STABILIZER' FOR OSCILLATORS Filed Aug. 30, 1944 4 Sheets-Sheet1 OSCILLATQR 2 LOAD FIGJ p 7, l2. e l

I u OSCILLATOR Z LOAD FIGZ / u f' e 4- OSCILLATOR a LOAD 3/ FIQQ 23/-'24 OSCILLATOR v I LOAD FIG-.4

INVENTOR. WILLIAM E. BRADLEY ATTORNEY Oct. 18, 1949. w. E. BRADLEY2,485,029

FREQUENCY STABILIZER FOR OSCILLATORS Filed Aug. 30, 1944 4 Sheets-Sheet2 INVENTOR.

WILLIAM E. BRADLEY ATTORNEY Oct. 18, 1949.

Filed Aug. 30, 1944 W. E. BRADLEY FREQUENCY STABILIZER FOR OSCILLATOHSOSCILLATOR 4 SheetsSheet I5 OSCILLATOR LOAD LOAD

IN VEN TOR.

WILLIAM E. BRADLEY ATTORN EY Oct. 18, 1949.

W. E. BRADLE Y FREQUENCY STABILIZER FOR OSCILLATORS 4 Sheets-Sheet 4OSCILLATOR FIG. I2

OSCILLATOR Filed Aug. 50, l94-4 M T m w.

WILLIAM E. BRADLEY ATTORN EY FIG.I4

Patented Oct. 18, 1949 FREQUENCY STABILIZER FOR OSCILLATORS William E.

Philadelphia, Pa., a co vania Bradley, Swarthmore, Pa., assignor, bymesne assignments,

to Philco Corporation,

rporation of Pennsyl- Application August 30, 1944, Serial No. 551,951

11 Claims. 1

My invention relates to a frequency stabilizing system and moreparticularly relates to a system for adjusting and stabilizing thefrequency of high frequency oscillators as the voltage, current or loadimpedance varies.

In various applications of oscillators using electronic tubes, itbecomes desirable at times to Vary the load on the oscillator.Unfortunately, a variation in load is usually accompanied by a variationin the oscillator frequency. Such a variation in frequency may beespecially serious in some types of radio communication systems in whicha transmitter and a receiver, remotely disposed from each other, must bekept synchronized so that the difference between their frequencies mustbe maintained at a constant within a specified tolerance.

One method of isolating the load from the oscillator consists ofinterposing an amplifier between them. This well known bufferarrangement for preventing changes in frequency caused by load changesis, however, undesirable or impossible in certain applications ofoscillators. For example, in the micro-wave region, where magnetronoscillators may be made to give very large instantaneous power outputsas oscillators, but do not function conveniently as amplifiers, such abuffer is impractical.

In accordance with my invention, frequency stabilization is effected byreflecting at predetermined points on the output transmission line ofthe oscillator in response to a shift in frequency from normal, theproper kind of reactance into the oscillator circuit.

In accordance with my invention, frequency stabilization is effected byreflecting a reactance into the line in response to a shift in frequencyfrom normal of a kind that will tend to restore the frequency to itsoriginal value.

More specifically, in one form of my invention I connect a parallelreactance resonant to the frequency to be controlled across thetransmission line. There are a number of points along the transmissionline one half wave length apart where an inductive reactance applied tothe transmission circuit will elevate and a capacitative reactance willlower the frequency of the oscillator. At one of these points where thiseffect is maximum, I shunt my parallel resonant circuit across thetransmission line which matches the impedance of the load.

It now the load impedance so changes that the frequency rises, acapacitance is presented by the parallel resonant circuit to thetransmission line which lowers the frequency to its normal value.

If the load impedance so changes that the frequency falls, inductance ispresented in parallel with the transmission line which raises thefrequency to its normal value, by lowering the net inductance.

Halfway between the points along the transmission line referred to abovebecause of the impedance transformation properties Of a transmissionline, an inductive reactance will lower and a capacitative reactancewill elevate the frequency. This reactance is secured by a resonantseries inductance and capacitance connected in series in the system atone of these half way points.

If the frequency rises in response to a change in load impedance abovethe stabilizing value and therefore above resonance frequency, aninductance is presented by the series resonant circuit to thetransmission line which lowers the frequency to its normal value.

If the load impedance .so changes that the frequency falls, capacitanceis presented to the transmission line which raises the frequency to itsnormal value.

In the discussion above, I have referred to a special network. This,,aswill be clear from the description to follow, can also be a network inthe coaxial sense, i. e., an arrangement of coaxial transmission line orit may be an arrangement of wave guide sections or resonant cavities.Arrangements can also be made using combinations of network elements,coaxial lines, wave guides and resonant cavities as will be explainedhereinafter.

Accordingly, an object of my invention is to provide anovel networkarrangement for effecting frequency stabilization of a system.

A further object of my invention is to provide a novel reactance networkcoupled to a circuit, the reactance reflected into the oscillatingcircuit being a function of the frequency fluctuations from normal tomaintain the frequency of the system substantially constant.

Still another object of my invention is to provide a novel resonantparallel inductance capaci- I tance circuit so connected in atransmission system that it presents capacitance to the line when thefrequency rises to lower the frequency to normal and presents inductanceto the line when the frequency drops to raise the frequency to normal.

Another object of my invention is to provide a novel resonant seriesinductance capacitance circuit so connected in a transmission systemthat it presents inductance to the line when the frequency rises tolower the frequency to normal and presents capacitance to the line whenthe frequency drops to raise the frequency to normal.

Still a further object of my invention is to provide a novel frequencycontrolled stabilizer which tends to maintain the net load impedancesubstantially constant.

These and other objects of my invention will appear from a detaileddiscussion which follows in connection with the drawings, in which:

Figure 1 is a schematic circuit diagram for illustrating my invention.

Figure 2 is a schematic circuit diagram of one form of my invention.

Figure 3 is a schematic circuit diagram of another form of my invention.

Figure 4 is a circuit diagram of my invention using a parallelinductance capacitance resonant circuit.

Figure 5 shows a method of connecting coaxial lines in parallel.

Figure 6 shows a method of connecting coaxial lines in series.

Figure 7 shows the corresponding arrangement to Figure 6 applied to awave guide.

Figure 8 shows the corresponding arrangement of Figure 5 applied to awave guide.

Figure 9 shows the invention applied to a resonant cavity with a coaxialline.

Figure 9a. is the schematic equivalent of Figure 9.

Figures 10 and 11 show schematic circuit embodyin my invention.

Figure 12 shows in cross-section a specific construction of wave guideembodying my invention.

Figure 13 shows in cross-section a further specific construction of acoaxial line embodiment of my invention; and

Figure 14 shows a further development of my invention utilizing theseries coaxial line junction illustrated in Fig. 6.

In order to clarify the invention, a brief theoretical discussion of theprinciples here involved will first be given.

Referring to Figure 1, an oscillator l is shown connected to a load 2over conductors 3 and 4 which are matched to the load impedance. In thissystem a fluctuation in reactance of the load will effect acorresponding change in frequency of the system.

To stabilize the frequency and prevent this shift, I insert at a properpoint along the line in the system a special frequency stabilizingnetwork H as shown in Figure 3. This stabilizing series network havingan impedance Zn is connected in series with the load in the circuit at apoint where an inductive reactance most lowers the frequency and acapacitative reactance most raises the frequency. Such points exist atregular intervals one half wave length apart along the transmissionline.

At regular intervals one half wave length apart and half way between thepoints referred to above, there exists points where an inductivereactance reflected into the system most raises the frequency and acapacitative reactance most wers the frequency. At such points aparallel resonant circuit schematically illustrated in Figure 2 isinserted across the line.

In Figure 4 this is more fully shown by an inductor and a capacitor inparallel. Here the oscillator 2| is connected to the load 22 over acircuit across which there is connected a stabilizing inductor 23 andstabilizing capacitor 24 connected in parallel resonance.

This inductance and capacity tuned to resonance at the desiredstabilized frequency is connected across the transmission line as shownat a point where an inductive reactance most raises the frequency and acapacitatlve reactance most lowers the frequency. Such points exist atpoints along the line a half wave apart and half way between the pointshereinbefore described in connection with the series resonant circuits.

Normally with the line matched to the load, this resonant circuit has noeffect on the system so long as the frequency is at the desired value.If, however, the load impedance varies, causing a rise in the oscillatorfrequency, the parallel reactance 23, 24 presents a capacitativereactance to the circuit which lowers the frequency to normal. If theshift in load impedance cause the frequency to fall, the parallelreactance 23, 24 presents an inductive reactance to the circuit whichraises the frequency to normal.

One characteristic of the corrective network is that near the normalfrequency, the reactance or susceptance as the case may he, must varyquite rapidly with frequency. This means that the Q of these elementsmust be high for close regulating action.

Accordingly, to secure most powerful stabilization. the inductance 23and capacitance 26 are very small reactances, providing a good or aboutzero power factor. As a result, although the impedance is high atresonance, large corrective susceptances are rapidly thrown across theline for small changes of frequency.

As described above, it will be noted that with the device shown inFigure 3, if the frequency be forced by changes within the oscillator tofrequencies too far removed from the normal frequency, the reactance ofmy special network will be very great, either positive or negative.This, however, would act as an open circuit and would tend to disconnectthe load from the oscillator and cause the oscillator to operateunloaded. If the oscillator can operate in one mode only, this causes notrouble, but if the oscillator has two or more modes of oscillation, i.e., two or more frequencies of oscillation, it may elect to oscillate ina mode where there is no load. A similar situation would result in theparallel case of Figure 4 in which off resonance my special networkbecomes practically a short circuit. A magnetron is particularlysusceptible to this type of difiiculty.

In order to overcome this unloading eifect, I place a resistor 8| inshunt with the tuned circuit 82, 83 of Figure 10 (the final form ofFigure 3) in order that even though the tuned circuit 82, 83 became anopen circuit, the load is connected to the oscillator through theresistor 8|. The final form of Figure 4 is shown in Figure 11, in whicha resistor 9| is placed in series with the resonant circuit 92, 93 sothat the load is never completely short circuited. With thesearrangements, the oscillator can never operate unloaded. and the rapidchange of reactance or susceptance of the special network with rroquencychange is not lost.

These principles of stabilisation can also be applied to systems usingmicro-wave frequencies. When the conductors between the oscillator andload are replaced by a coaxial transmission line, a modification ofFigure4 can be made as is shown in Figure 5. The coaxial line 81, 32conducts power from the oscillator to the load. At such a position onthis line that the addition of a. capacitance lowers the frequency amaximum amount, a stub line 33, which is an odd number, of quarter waveslong, is inserted at right angles to and in electrical parallelconnection with the main line.

One end of stub line 33 is open and fits into an opening in the outershield of coaxial line it. The inner conductor of the stub extendsthrough the junction of the stub and main coaxial line and is connectedto the inner conductor of the main coaxial line. The opposite end of thestub is closed oflf at 34, so that it presents an infinite impedance tothe main line 3|. !2 at the normal frequency. If now the load is changedso that the frequency tends to decrease, this stub line operates as aless than quarter wave line, and thus operates to add inductance to theline and therefore to raise and restore the frequency to its normalvalue.

Because of its position on the line, its action counteracts the changein load impedance. Conversely, if the load impedance so changes that itacts as an inductance at the junction point of the main line and stuband the frequency rises, the stub line operates as a greater thanquarter wave line and thus adds capacitance to the line causing thefrequency to restore itself.

A coaxial counterpart of Figure 3 is shown in Figure 6. The centralconductor 4| passes straight through the system. The outer conductor 42of the main transmission line formed by 4| and 42 is broken at 43 at apoint where a series inductance introduced into the transmission linedecreases the frequency and a capacitance introduced into the lineraises the freuency.

This break leads into a section of transmission line with the outersurface of 42 acting as the inner conductor and with a hollowcylindrical tube 44 acting as the outer conductor. The ends 35 and 46 ofthe tube are closed and in contact with conductor 42 along thecircumferences 41 and 48 respectively.

The length of this section of 46 to 4=l is chosen to be one half wavelength or any other integral number of halves .of a wave length of thefrequency to be stabilized. This means that the series impedance lookinginto this section of the line from the main transmission line issubstantially zero. Consequently, there results an effectivelycontinuous transmission line at 43 when the stabilizing frequencyexists.

At the frequency to be stabilized, the stabilizing unit does not haveany effect upon a transmission through the main transmission line fromthe oscillator to the load.

If an impedance of the load is changed so that the impedance lookinginto the main :section of the transmission line at position 43 appearsto have a capacitance added to it, the frequency of the oscillatorrises.

As the frequency begins to increase, the impedance looking into thecompensator no longer appears the same as that-of a similar line withoutthe compensator, as a short circuit; instead it appears to be aninductlmce. This inductance acts in series with the equivalentcapacitance seen looking towards the load and consequently tends toneutralize this capacitance. The net result of this action is to tend tohold the oscillator frequency substantially at a predetermined value.

correspondingly, and for reasons that now will be clear, when thefrequency goes below the predetermined value, a. corrective capacitanceto increase the frequency is applied to the line.

A form of Figure 3 using a series wave guide element is shown in Figure'7. Here the main wave guide 5| is a hollow tube of metal of rectangularcross-section, designed to carry the transverse electric mode with thelines of electric field parallel to the narrow dimension of the guide.The branch guide 52 of similar rectangular cross-section is an integralnumber of half wave lengths long opening into a, cut in the largedimension or the main guide at the junction 54 and is closed at the end'53.

All the longitudinal current in the main guide must then flow in thebranch guide and this arrangement is spoken of as a series connection.The branch guide 52 is connected to the main guide at a point where theaddition of inductance lowers the frequency and the addition ofcapacitance raises the frequency.

A form of Figure 5 using a shunt wave guide section an odd number ofquarter waves length long is shown in Figure 8. Here the main wave guideBI is jointed at 62 to a branch guide 68 at a cut in both along thenarrow edge of the guide. The branch guide is closed at its end 64. andthe operation is entirely analogous to the operation of the coaxialsystem described in connection with Figure 5.

A combination of a resonant cavity with a coaxial line is shown inFigure 9. Here the main line 1| is broken by a slit 12 in the outerconductor. A cavity resonator I3 is fastened on the outside of this slitand serves the same function as the external line section in Figure 5.To accomplish this the cavity must be so dimensioned, in accordance withprinciples well known to the art, that it resonates in a mode such thatit appears as a zero impedance in series with the line at the resonantfrequency.

While the-embodiments shown in Figures 5, 6, 7, 8 involve the principleof the invention, there are times when a more powerful stabilizingaction is required. For this purpose the reactamoe change with frequencyof the stabilizing network should be relatively great. This is obtainedby having an exceedingly low L to 0 ratio in the tuned circuit of Figure11 and an exceedingly high L to C ratio for the tuned circuit of Figure10.

It is inconvenient to build straightforwardly constructed tuned circuitshaving the required L to C ratio, but over a narrow band the desiredperformance can readily be obtained by means of other types of networks,such as coaxial lines. resonators or wave guides.

A specific arrangement of my invention using wave guides is shown inFigure 12 in cross section. This is a modification of Figure 7, thecross section being taken through the narrow width of the wave guide.The coupling junction fill of the branch guide N12 to the main guide M3is at such a point along the main guide that an increase in theeffective capacitance of the load as viewed from this junction pointwould cause the frequenc to rise. .At the neutral .frequency, the

branch guide system presents a short moon. to

the main guide at junction I M In order that this may be so, thedistance from junction IM to junction I04 is made close to a quarterwave length. An open circuit at junction I04 would appear as a shortcircuit at junction IOI because of the well known impedance inversionproperties of a quarter wave length of wave guide. At junction I04 thetwo sub-branches I05 and I06 are in series because they are joined alongtheir broad faces. The impedance looking into sub-branch I05 is aresistance, because I05 is partially filled with a lossy material I01which absorbs any energy which enters I -5 at I04. As viewed from I04,sub-branch I06 appears to have whatever impedance is presented to it atjunction I08 by cavity resonator I09, because of the well knownimpedance retaining property of the half wave length section of guidebetween I04 and I08. At the normal frequency, this cavity resonator istuned by adjustin screw IIO, which may be placed almost anywhere in thewall of the cavity, so that the impedance of the cavity as seen fromjunction I08 is infinite. Then the impedance of section I 08 as seenfrom I04 is infinite, and the series addition of I is of no importance.

Consequently, the impedance as seen from junction ml is zero. When thefrequency is slightly raised, the impedance transformation properties ofthe wave guide sections do not change very much. However, the impedanceof the cavity becomes a high capacitive reactance, which then places ahigh capacitative reactance in series with the lossy subbranch I05 atjunction I04. Again, the impedance of the lossy subbranch isinsignificant compared to the impedance in series with it, so theimpedance as viewed from junction IOI becomes an inductive reactancebecause of the impedance inversion qualities of a quarter wave guidesection. This is just what is needed in order that the regulatory actiondescribed in connection with Figure 2 may become effective. Atfrequencies far removed from normal, the impedance of the cavity asviewed from I08 becomes low, so the impedance as viewed from junctionI04 becomes substantially the impedance of the lossy subbranch I05.Consequently, a resistive impedance is presented to the main guide at MIand the load is not disconnected, as it would be if the lossy guidesection I05 were absent.

Another specific arrangement of my invention is described with referenceto Figure 13. The main transmission line I2I and I22 is of the coaxialtype, with an outer conductor I2I shown in section, and an innerconductor I22. A branch system is joined to the main coaxial line at ajunction point I24 which is so chosen that an increase in the apparentinductance of the load as viewed from the junction point would cause thefrequency to increase at a maximum rate. At the normal frequency, theimpedance looking into the branch section at junction I24 is infinite.

In order for this to be true, the impedance at junction I25 which is thejunction between a lossy section of line I26 and a cavity resonator I21,must be infinite because the length of line between junctions I24 andI25 is a half wave length. The impedance looking into the lossy sectionof line is primarily resistive, and is in series with the impedancelooking into the cavity resonator from I 25. This cavity resonatorimpedance is made infinite by adjustment of the tuning screw I28. Thusthe impedance of the branch at junction point I24 is infinite.

At resonant frequency, the cavity looks like a very high impedance inseries with the line.

8 Since the cavity is a half wave from the main transmission line whenit appears as a nearly open circuit, the stub looks substantially likean open circuit to the transmission line. When, however, the cavitylooks like a relatively low impedance, i. e., when oil resonantfrequency, the low value of impedance is limited by series mismatchingresistor 1', which prevents the stub from ever short-circuiting the maintransmission line.

When the frequency is slightly raised, the impedance of a cavity coupledin this manner becomes a capacitive reactance, as is well known inmicro-wave theory, and so the impedance in parallel with the line at I24becomes capacitive, which is what is required for the regulatory actiondescribed for Figure 4 to take place.

When the frequency is far from the cavity resonant frequency, theimpedance of the cavity is low, so the net impedance at junctions I24and I25 is the resistance of the lossy line. This preventsshort-circuiting of the load at frequencies far away from resonance, andthus acts to prevent oscillator operation at a frequency of any otheroscillation mode.

Figure 14 is a coaxial line embodiment of Figure 10 and is noteworthyfor its compactness. The cavity and resistive material are arranged in amanner electrically similar to the structure of Figure 13 except thatthe cavity is an odd number of quarter waves from the junction with themain line. which is of the type shown in Figure 6.

In Figure 14 the stubitself however is more complex than the one inFigure 6. The cavity resonator is coupled to the stub at a distance fromthe series junction I43 equal to an odd number of quarter wave lengthsof the frequency to be stabilized. The cavity I44 is coupled to the studline through a slot I45. The remaining part of the stub line is loadedat its end with some lossy material I46.

The cavity is coupled to the line section in physically the same butelectrically a different way than the cavity I3 is coupled to line II inFigure 9. This electrical difierence arises from the fact that now thecavity is resonating with the electrical lines parallel to the axis ofthe cavity.

The resonant cavity I3 is magnetically coupled to the transmission line,1 I. The electric coupling cancels out over the whole interior space andaccordingly it acts as if it were coupled purely by inductance at theresonant frequency of the tuned circuit shown in Figure 9a.

A very high impedance limited only by losses in the circuit appearsacross the terminals I and 2 of Figure 90. At this same resonantfrequency a very high impedance also appears across the terminals 3 and4. However, it is only across a narrow band that the impedance acrossthe terminals 3 and 4 is similar in form to that across I and 2. Thisimpedance between terminals 3 and 4 diiiers in two particulars from thatacross I and 2. In the first place, it appears to be the impedance of atuned circuit of vastly greater 0 to L ratio than the impedance betweenI and 2. The apparent 0 to L ratio depends on the proportion ofinductance included between 3 and 4. The larger this inductance the lessthe C to L ratio. On the other hand. it difiers also in the particularthat at some higher frequency the impedance between 8 and 4 goes tozero. This frequency is so far removed in the actual designs of cavitiesused as to be immaterial and the device is used only in the neighborhoodof W0 which is the resonant frequency of the tuned circuit.

In this mode or cavity resonance, the structure Figure 9 therefore hasthe property that when the cavity 13 is in a resonant condition thecoaxial line is completely open-circuited opposite the slot 12.

The result in Figure 14 is that at the junction I45 the cavity presentsa high resonant impedance in series with the impedance of the rest ofthe stub which is a resistive impedance. At the frequency to bestabilized, the resonant impedance of the cavity is infinite, so theseries resistance of the lossy stub is of no consequence.

In Figure 14, the transformation action caused by the odd quarter wavelength section of line makes the impedance of the stub at the seriesjunction I43 appear as a short-circuit at the frequency to bestabilized. If, however, the load of the main line is changed so thatthe change in load causes the impedance looking towards the load fromjunction I43 to become capacitive, the frequency of the oscillator willrise because the series junction I43 is so placed on the maintransmission line that this will occur.

This rise in frequency causes the resonant impedance of the cavity tobecome less than infinite and to become capacitive. It is still high,however, and consequently the impedance corresponding to the increase infrequency causes the impedance at the junction M5 to become capacitive.At this junction, the series resistance of the lossy section of the stubis negligible. This impedance when transformed by the odd quarter wavesection of transmission line appears as a low inductive reactance at theseries junction I43. This inductive reactance then compensates for thecapacitive reactance of the load as seen from the location I43. As aconsequence, the frequency need change only by the amount necessary forthis compensation to occur.

If now the oscillator should attempt to oscillate at a frequency farremoved from the desired operating frequency, the impedance of thecavity would become low. This impedance in series with the impedance ofthe lossy section of the line presents substantially a resistiveimpedance to the end of the odd quarter wave length section of stubline. The transformation property of this odd quarter wave lengthsection is such that there will appear at the series junction aresistive impedance looking into the stub. This impedance will be inseries with the load impedance and consequently the oscillator willstill be loaded. This will in effect prevent the oscillator fromoperating at an undesired mode.

If the lossy section of stub line had not been present, the impedance atseries junction I43 under these circumstances are removed from theresonant frequency of the cavity; that is, from the desired operatingfrequency would become an open circuit. This open circuit wouldeffectively disconnect the load from the oscillator and might lead theoscillator to operate at a frequency far removed from the desiredfrequency. Not all oscillators, of course, have this difficulty, butmagnetrons in particular do have a tendency to oscillate at a secondfrequency if they are unloaded at that frequency. The function of thelossy sec tion will in that case prevent such undesirable oscillationfrequencies.

Experimental work with frequency stabilizers of this type indicate thatthey are also effective in reducing the frequency change caused byvariations in voltage or current in the oscillator tube. They findapplication in radar systems where reflected waves, such as may comefrom the antenna 10 housing, effect an impedance change in the antennaload.

Various modifications of the principles of my invention will now beevident to those skilled in the art. I therefore prefer not to be boundby the specific disclosures hereinabove set forth, but only by theappended claims.

I claim:

1. In a micro-wave electrical system having a transmission line for thetransmission of energy in the microwave region, a magnetron oscillatorhaving two or more modes of oscillation coupled to said line, a loadconnected to said line, said line having a number of points therealong,one half Wave length apart where reactance can be reflected into theline in response to a shift in frequency from a predetermined value torestore the frequency to its predetermined value, a reactance connectedto said line at one of said points for effecting the frequencies of saidsystem, and a resistance in circuit connection with said reactance forsuppressing modes of oscillations other than the desired mode.

2. In a micro-wave electrical system having a transmission line for thetransmission of energy in the micro-wave region, a magnetron oscillatorhaving two or more modes of oscillation coupled to said line, a loadconnected to said line, there being a number of points along saidtransmission line one half wave length apart, where an inductivereactance applied to the line will elevate and a capacitative reactanceapplied to the line will lower the frequency of said oscillator, areactance shunted across said line at a point where its effect ismaximum for effecting the frequencies of said system, said rectancematching the impedance of said load and presenting capacitance to. theline, if the frequency rises, to lower the frequency to itspredetermined value and presenting inductance in parallel to the line ifthe frequency falls below said predetermined value to restore saidfrequency to said predetermined value and a resistance connected inseries with said reactance for suppressing modes of oscillation otherthan the desired mode.

3. In short wave electrical system having a transmission line for thetransmission of short wave energy, a magnetron oscillator having two ormore modes of oscillation coupled to said line, a load connected to saidline, said line having a number of points therealong, one half wavelength apart where reactance can be reflected into the line in responseto a shift in frequency from a predetermined value to restore thefrequency to its predetermined value, a reactance comprising aninductance and capacitance of relatively small reactance value providingsubstantially zero power factor connected to said line at a point whereits effect is maximum to reflect a substantially large correctivereactance into the system in response to small change in frequency ofsaid oscillator from a predetermined frequency for maintaining thefrequency thereof constant as it tends to vary from a predeterminedvalue and a resistance in circuit connection with said reactance forsuppressing modes of oscillation other than the desired mode.

4. In a short wave electrical system having a transmission line for thetransmission of short wave energy, a magnetron oscillator having two ormore modes of oscillation coupled to said line, a load connected to saidline, there being a num ber of points along said transmission line onehalf wave length apart, where an inductive reactance applied to the linewill elevate and a capacitative reactance applied to the line will lowerthe frequency of said oscillator, a reactance comprising an inductanceand capacitance oi relatively small reactance value providingsubstantially zero power factor connected to said line at a point whereit reflects a substantially large corrective inductive reactance intothe system in response to small changes in frequency of said oscillatorfrom a predetermined frequency for elevating the frequency thereof tonormal in response to a drop in frequency and a resistance in circuitconnection with said reactance for suppressing modes of oscillationother than the desired mode.

5. In an electrical system having a transmission line, a magnetronoscillator having two or more modes of oscillation coupled to said line,a load connected to said line, there being a number of points along saidtransmission line, one half wave length apart, where an inductivereactance reflected into the line will lower and a capacitativereactance reflected into the line will raise the frequency of saidoscillator, a reactance comprising an inductance and capacitance inwhich the L to C ratio is high, connected to said line at such a pointwhere it reflects a substantially large corrective inductive reactanceinto the system for lowering the frequency thereof to normal in responseto a rise in frequency, and a resistance connected across said reactancefor suppressing modes of oscillation other than the desired mode.

6. In an electrical system having a transmission line, a magnetronoscillator having two or more modes of oscillation coupled to said line,a load connected to said line, there being a number of points along saidtransmission line, one half wave length apart, where an inductivereactance applied to the line will elevate and a capacitative reactanceapplied to the line will lower the frequency of said oscillator, areactance comprising an inductance and capacitance in which the L to Cratio is relatively very low connected to said line at such a pointwhere it reflects substantially large corrective capacitance reactanceinto the system in response to small changes in frequency of saidoscillator from a predetermined frequency for lowering the frequencythereof to normal in response to a rise in frequency and a resistanceconnected in series with said reactance for suppressing modes ofoscillation other than the desired mode.

'7. In an electrical system having a transmission line, a magnetronoscillator having two or more modes of oscillation coupled to said line,a load connected to said line, there being a number of points along saidtransmission line, one half wave length apart, where an inductivereactance reflected into the line will lower and a capacitativereactance reflected into the line will raise the frequency of saidoscillator, a reactance comprising an inductance and capacitance inwhich the L to ratio is high connected to said line at such a pointwhere it reflects a substantially large capacitance reactance into thesystem for elevating the frequency thereof to normal in response to adrop in frequency, and a resistance connected across said reactance forsuppressing modes of oscillation other than the desired mode.

8. In short wave electrical system having a transmission line for thetransmission of shortwave energy, a magnetron oscillator having two ormore modes of oscillation coupled to said line, a load connected to saidline, there being a number of points along said transmission line onehalf wave length apart, where an inductive reactance applied to the linewill elevate and a capacitative reactance applied to the line will lowerthe frequency of said oscillator, a circuit for stabilizing thefrequency of said oscillator comprising parallel inductance andcapacitance whose L to C ratio is low connected across said line at apoint where capacitance most lowers the frequency in response to a riseof frequency, said inductance and capacitance being tuned to resonate atthe frequency to be stabilized, the reactance of said inductance andcapacitance being small compared to the impedance of said transmissionline and a resistance connected in series with said reactance forsuppressing modes of oscillation other than the desired mode.

9. In short wave electrical system having a transmission line for thetransmission of short wave energy, a magnetron oscillator having two ormore modes of oscillation coupled to said line, a load connected to saidline, there being a number of points along said transmission line onehalf wave length apart, where an inductive reactance applied to the linewill elevate and a capacitative reactance applied to the line will lowerthe frequency of said oscillator, a circuit for stabilizing thefrequency of said oscillator comprising parallel inductance andcapacitance whose L to C ratio is low connected across said line at apoint where capacitance most lowers the frequency in response to a riseof frequency, said inductance and capacitance being tuned to resonate atthe frequency to be stabilized, and a resistance connected in serieswith said parallel inductance and capacitance of a value for suppressingmodes of oscillation other than the desired mode.

10. In short wave electrical system having a transmission line for thetransmission of shortwave energy, a magnetron oscillator having two ormore modes of oscillation coupled to said line, a load connected to saidline, said line having a number of points therealong, one-half wavelength apart where reactance can be reflected into the line in responseto a shift in frequency from a predetermined value to restore thefrequency to its predetermined value, a circuit for stabilizing thefrequency of said oscillator comprising a series inductance andcapacitance whose L to 0 ratio is high and tuned to resonance at thefrequency to be stabilized and connected in said line at a point whereit reflects a corrective impedance into said line in response to achange in frequency from normal for restoring said frequency to itsnormal value and a resistance in circuit connection with said reactancefor suppressing modes of oscillation other than the desired mode.

11. In short wave electrical system having a transmission line for thetransmission of shortwave energy, a magnetron oscillator having two ormore modes of oscillation coupled to said line, a load connected to saidline, said line having a number of points therealong, one half wavelength apart where reactance can be reflected into the line in responseto a shift in frequency from a predetermined value to restore thefrequency to its predetermined value, a circuit for stabilizing thefrequency of said oscillator comprising a series inductance andcapacitance whose L to C ratio is high and tuned to resonance at thefrequency to be stabilized and connected in said line at a point whereit reflects a corrective impedance into said line in response to achange in WILLIAM E. BRADLEY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PA'I'ENTS Number Name Date 1,486,506 Wagner Mar. 11, 19241,813,488 Field July 7, 1931 Number 14 Name Date Kummerer Aug. 18, 1941Conklin Jan. 1, 1935 Briggs July 27, 1937 Schelkunoff Apr. 25, 1939Dallenbach Apr. 30, 1940 Alford Apr. 15, 1941 Jakel Dec. 23, 1941 BarrowMay 5, 1942 Salinger June 8, 1942 Dow Apr. 10, 1945 OTHER REFERENCESRadio, July 1944, pages 22-26 and '76.

